Tensile and Charpy impact properties of high-strength bainitic steels fabricated by controlled rolling process

This study is concerned with tensile and Charpy impact properties of high-strength bainitic steels fabricated by controlled rolling process. Six kinds of steels were fabricated by varying finish rolling temperature, start cooling temperature, and cooling rate, and their microstructures and tensile and Charpy impact properties were investigated. Their effective grain sizes were also characterized by the electron backscatter diffraction analysis. The microstructures of the steels rolled in the single phase region were most similar to those of the steels rolled in the two phase region. The steels cooled from 700°C were composed mainly of granular bainites, while those cooled from 600°C contained a number of bainitic ferrites, which resulted in the decrease in ductility and upper shelf energy in spite of the increase in strength. In the steels cooling from 600°C, fine acicular ferrites were well formed when the cooling rate was slow, which led to the best combination of high ductility, high upper shelf energy, and low energy transition temperature according to the decrease in the overall effective grain size due to the presence of acicular ferrites having smaller effective grain size.